As the field of electronics has grown, so has its involvement with musical instruments. One case in point is the electric guitar. The early hollow body electric guitars were introduced at the beginning of the 1930's. Within a decade the solid body electric guitar, most familiar today, would follow. Solid body electric guitars lack the acoustic chamber to amplify the sound of the strings. Consequently, they produce relatively little sound themselves. In order to resolve this problem electric guitars relied on “pickups.” The pickup was a passive solid state device embedded in the body of the guitar beneath the strings. It contained a series of magnets wrapped in very fine wire. Each magnet corresponded to a string and when the string vibrated, that string would induce a current, proportional to that vibration, in the wire around the magnet. This current would then be transmitted via a cable which ran from the guitar to an amplifier. The amplifier would then step up the original signal to produce an amplified sound that would accord with the sound of the strings on the guitar.
This early design of the electric guitar, still available today, did pose certain technical problems. One of these problems was the physical inconvenience of the cable that ran from the guitar to the amplifier, particularly when performing musicians moved around on a platform or stage. The cable could become tangled, providing a safety hazard, or pull loose from either the guitar or amplifier interrupting the signal and the performance. This led to the introduction of wireless transmitters that could be attached to the guitar.
Another problem lay in the pickups themselves. By their very nature pickups were susceptible to magnetic flux. Magnetic pickups tend to pick up ambient, and usually unwanted, electromagnetic noises. The resulting noise, the so-called “hum”, is particularly strong with single-coil pickups, and aggravated by the fact that very few guitars are correctly shielded against electromagnetic interference. A common cause is the relatively strong 50 or 60 Hz electromagnetic noise associated with the distribution of conventional line voltage (e.g., 110 or 220 Volt AC). As nearly all amplifiers and audio equipment associated with electric guitars rely on this power, there is in theory little chance of completely eliminating the introduction of unwanted hum. One attempted solution was the introduction of so called “active pickups.” This innovation essentially incorporated an electronic pre-amp into the pickup in order to compensate for the extraneous electromagnetic influences and provide a better, cleaner sound.
Both wireless transmitters and active pickups are examples of self contained electronic devices incorporated into the instrument itself. This required a new element in a now truly electric guitar: an independent power source. In order to power these electronic devices, batteries are carried by the musician or implanted in the instrument. These batteries are typically of differing voltages and require periodic replacement or, as may be preferred, recharging. In keeping with the benefits of the modern autonomous electric instrument it is desirable that this be accomplished while maintaining that autonomy and consequently the greatest possible convenience for the musician. Some methods have been proposed such as forming an electrical circuit using metal contacts. One problem with this approach is that the contact points may be degraded by deposits or oxidation and fail to make a reliable connection. It is also undesirable both aesthetically and for the physical and tonal integrity of an instrument to make multiple penetrations into the instrument in order to charge multiple batteries. It would therefore be desirable to find an unobtrusive and minimally intrusive means of recharging one or more batteries of varying voltages in an electric musical instrument without disconnecting the batteries from the instrument, making multiple penetrations, or requiring the use of a fixed cable.